Batteries used in stand alone power supply systems are commonly lead-acid batteries. However, lead-acid batteries have limitations in terms of performance and environmental safety. For example, typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and presents environmental challenges during manufacturing and disposal.
Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer the potential to overcome the above mentioned limitations of lead-acid batteries. In particular, the operational lifetime of flowing electrolyte batteries is not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
A flowing electrolyte battery, like a lead acid battery, comprises a stack of cells that produce a total voltage higher than that of individual cells. But unlike a lead acid battery, cells in a flowing electrolyte battery are hydraulically connected through an electrolyte circulation path.
Referring to FIG. 1, a flow diagram illustrates a basic zinc-bromine flowing electrolyte battery 100, as known according to the prior art. The zinc-bromine battery 100 includes a negative electrolyte circulation path 105 and an independent positive electrolyte circulation path 110. The negative electrolyte circulation path 105 contains zinc ions as an active chemical, and the positive electrolyte circulation path 110 contains bromine ions as an active chemical. The zinc-bromine battery 100 also, comprises a negative electrolyte pump 115, a positive electrolyte pump 120, a negative zinc electrolyte (anolyte) tank 125, and a positive bromine electrolyte (catholyte) tank 130. A complexing agent is generally added to the bromine electrolyte to form a polybromide complex that reduces the reactivity and vapour pressure of elemental bromine.
To achieve high voltage, the zinc-bromine battery 100 further comprises a stack of cells connected in a bipolar arrangement. For example, a cell 135 comprises half cells 140, 145 including a bipolar electrode plate 155 and a micro porous separator plate 165. The zinc-bromine battery 100 then has a positive polarity end at a collector electrode plate 160, and a negative polarity end at another collector electrode plate 150.
A chemical reaction in a positive half cell, such as the half cell 145, during charging can be described according to the following equation:2Br−→Br2+2e−  Eq. 1Bromine is thus formed in half cells in hydraulic communication with the positive electrolyte circulation path 110 and is then stored in the positive bromine electrolyte tank 130. A chemical reaction in a negative half cell, such as the half cell 140, during charging can be described according to the following equation:Zn2++2e−→Zn  Eq. 2A metallic zinc layer 170 is thus formed on the collector electrode plate 150 in contact with the negative electrolyte circulation path 105. Chemical reactions in the half cells 140, 145 during discharging are then the reverse of Eq. 1 and Eq. 2.
All batteries employing aqueous electrolyte solutions will produce some hydrogen gas and hydroxide ions due to the electrolysis of water. If the hydrogen is allowed to escape the system then the pH of the electrolyte will eventually rise to the point where solid deposits can precipitate resulting in sub-optimal battery performance.
To combat this effect recombinator devices may be used to return the gaseous molecular hydrogen back into the circulating electrolyte stream as hydrogen ions and so prevent significant pH rises. Recombinators employ precious metal catalysts, such as platinum, to accelerate the reaction of hydrogen and bromine gases to form hydrobromic acid, thereby re-acidifying the system. The recombinator will be in fluid communication with a gas handling unit which receives gases produced in the electrode stacks and monitors and controls gas pressures.
When employing a recombinator great care must be taken to ensure the precious metal catalyst contained therein is unable to enter the electrolyte stream, which would lead to the catalyst plating out on the zinc electrodes and thus reducing battery performance and leading to loss of hydrogen ions. Further, the catalyst is generally maintained at elevated temperatures during operation to ensure it stays dry for optimal continuing performance. If the catalyst is subsequently allowed to cool in the presence of reactants, for example when the battery is not in use, it will continue producing hydrobromic acid, albeit at a lower rate. Eventually the catalyst will become saturated with this acidic liquid which can damage the catalyst resulting in a loss of performance and entry of catalyst material into the electrolyte stream.
A further problem with prior art recombinators is that the rate and efficiency of the return of hydrogen ions back into the system is generally limited by the availability of bromine for reaction. As mentioned above, the bromine in the system is usually complexed with a suitable complexing agent to reduce its vapour pressure, thereby resulting in a low bromine partial pressure in the system. However, that can lead to an insufficient amount of bromine to achieve an acceptable reaction rate within the recombinator.